The search for new therapeutic agents has been greatly aided in recent years by better understanding of the structure of enzymes and other biomolecules associated with target diseases. One important class of enzymes that has been the subject of extensive study is the protein kinases.
Protein kinases mediate intracellular signal transduction by effecting a phosphoryl transfer from a nucleoside triphosphate to a protein acceptor that is involved in a signaling pathway. There are a number of kinases and pathways through which extracellular and other stimuli cause a variety of cellular responses to occur inside the cell. Examples of such stimuli include environmental and chemical stress signals (e.g. osmotic shock, heat shock, ultraviolet radiation, bacterial endotoxin, H2O2), cytokines (e.g. interleukin-1 (IL-1) and tumor necrosis factor alpha (TNF-alpha)), and growth factors (e.g. granulocyte macrophage-colony-stimulating factor (GM-CSF), and fibroblast growth factor (FGF). An extracellular stimulus may effect one or more cellular responses related to cell growth, migration, differentiation, secretion of hormones, activation of transcription factors, muscle contraction, glucose metabolism, control of protein synthesis and regulation of cell cycle.
Many diseases are associated with abnormal cellular responses triggered by protein kinase-mediated events. These diseases include autoimmune diseases, inflammatory diseases, neurological and neurodegenerative diseases, cancer, cardiovascular diseases, allergies and asthma, Alzheimer's disease or hormone-related diseases. Accordingly, there has been a substantial effort in medicinal chemistry to find protein kinase inhibitors that are effective as therapeutic agents.
In humans, there are three highly related Aurora kinases that are all serine/threonine protein kinases (see Andrews, P. D., et al., Curr. Opin. Cell. Biol. 2003, 15, 672-683; Carmena, M., Earnshaw, W. C., Nat. Rev. Mol. Cell. Biol. 2003, 4, 842-854; Brown, J. R., et al., BMC Evol. Biol. 2004, 4, 39, Andrews, P. D., Oncogene 2005, 24, 5005-5015). Despite the sequence relatedness of Aurora A, B and C, the localization and function of these kinases is quite distinct. As a result, overexpression or activation of each of these kinases can be associated with different disease states, including proliferative diseases such as cancer.
Members of the family demonstrate distinct subcellular localization during mitosis and are degraded by the proteosome following exit from mitosis (Graham et al. (2002) J. Biol. Chem. 277:42419-22). The kinases are often found complexed with other proteins, including cytoskeletal structures.
The Aurora kinases share a conserved C-terminal catalytic domain, with greater variation being observed at the N-terminus. Aurora A (Aurora-2) is unique in the presence of two lysine residues in the nucleotide-binding domain of the kinase (Warner et al. (2003) Molecular Cancer Therapeutics 2:589-95).
Maximum levels of the Aurora A polypeptide, and maximum Aurora A activity, are observed at the G2/M transition leading into mitotic prophase, with the polypeptide localizing to the mitotic spindle pole (Graham et al. (2002) J. Biol. Chem. 277:42419-22; Sakai et al. (2002) J. Biol. Chem. 277:48714-23).
Aurora A appears to regulate chromosome duplication with aberrant expression being associated with aneuploidy and an aggressive clinical phenotype, particularly in solid tumors. Such observations, and additional experimental data, suggest that Aurora A disrupts the signaling cascade that regulates chromosome segregation (Sen et al. (2002) J. Nat. Cancer. Inst. 94:1320-29).
Aurora A also appears to function in meiosis, likely in separating homologous chromosomes and in spindle rotation. Injection of antibodies against Aurora A into Xenopus oocytes prevents first polar body extrusion and causes arrest at meiosis I (Castro et al. (2003) J. Biol. Chem. 2236-41). The Xenopus kinesin-like protein, Eg5, is known to be a substrate for Aurora-2 (Castro et al. (2003) J. Biol. Chem. 2236-41).
In addition, in vitro studies show that Aurora A is incorporated into chromatin and phosphorylates histone H3, and possibly histone H2B (Scrittori et al. (2001) J. Biol. Chem. 276:30002-10). H3 phosphorylation, e.g., at serine-10, during chromosome assembly, appears to be a conserved event in eukaryotic cell division. Inhibition of H3 phosphorylation leads to chromosome condensation, abnormal segregation, and the loss of chromosomes during mitosis and meiosis (Scrittori et al. (2001) J. Biol. Chem. 276:30002-10).
Accordingly, the emerging model for histone phosphorylation is analogous to that of histone acetylation, wherein partially redundant enzymatic activities are associated with histone modifications but different enzymes may function in different cellular contexts. For example, some enzymes may modify histones in bulk, while other enzymes modify histones in a targeted manner, i.e., in a sequence or domain-specific manner in the context of assembled chromatin (see, e.g., Scrittori et al. (2001) J. Biol. Chem. 276:30002-10). According to this model, Aurora A would appear to be a kinase responsible for targeted histone modification, in the context of assembled or assembling chromatin.
Other members of the Aurora kinase family are also associated with mitosis and meiosis. Aurora B, like Aurora A, is involved in distinct protein phosphorylation events that regulate the cell cycle. Unlike Aurora A, Aurora B is localized to inner-centromeric chromatin from prophase until the metaphase-anaphase transition, relocalizes to the microtubules in the spindle midzone during telophase, and subsequently is found in the midbody throughout cytokinesis (See Andrews, P. D., Oncogene 2005, 24, 5005-5015, loc. cit.). The function of Aurora B is to ensure accurate chromosome segregation and appropriate cytokinesis. Aurora B appears to associate with a survivin, a polypeptide that associates with the inner centromere and undergoes a significant degree of stretching during mitosis. Survivin appears to be involved with inhibition of apoptosis as well as cell cycle control. Interestingly, both Aurora B and survivin are delocalized during megakaryocyte endomitosis, a process by which late anaphase and cytokinesis are skipped, leading to megakaryocyte polyploidy (Zhang et al. (2004) Blood 103:3717-26). Inhibitors of this function in a proliferative disease such as cancer would lead to stasis and cell death, making such inhibitors useful in cancer chemotherapy.
Aurora C (Aurora-3) is the least studied, known member of the family. Aurora C localizes to centrosomes from anaphase until telophase (or even cytokinesis), and is highly expressed in the testis (Brown et al. (2004) BMC Evolutionary Biology 4:39).
As noted above, Aurora kinases are overexpressed in certain types of cancers, including colon, breast, and other solid-tumor cancers. The genes encoding the Aurora B and A kinases tend to be amplified in certain types of cancers, while the gene encoding the Aurora C kinase resides in a region of the chromosome that is subject to rearrangement and deletion. Aurora A has been associated with a variety of malignancies, including primary colon, colorectal, breast, stomach, ovarian, prostate, and cervical cancer, neuroblastoma, and other solid-tumor cancers (Warner et al. (2003) Molecular Cancer Therapeutics 2:589-95).
Inhibitors of Aurora A have been described. For example, Harrington et al. ((2004) Nat. Med. 10:262-67) have described VX-680, a small-molecule inhibitor that blocks cell-cycle progression and induces apoptosis in certain types of tumors in in vivo xenograft models. A pyrazole Aurora A kinase inhibitor is also described in U.S. Pat. No. 6,653,301 (Bebbington et al., issued Nov. 25, 2003).
Hauf et al. ((2003) J. Cell. Biol. 161:281-294) identified the indolinone (Hesperadin) as an inhibitor of Aurora B, which causes cells to enter anaphase with monooriented chromosomes, having both sister kinetochores attached to a single spindle pole (a condition known as syntelic attachment).
Ditchfield et al. ((2003) J. Cell Biol. 161:267-280) described ZM447-439 ((4-(4-(N-benzoylamino)anilino)-6-methoxy-7-(3-(1-morpholino)propoxy)quinazoline), an Aurora kinase inhibitor which interferes with chromosome alignment, segregation, and cytokinesis.
Accordingly, kinase inhibitors, particularly inhibitors of Aurora kinases, are of particular interest in treating certain disorders, including cancer. Compounds exhibiting such inhibition are of particular value.